MEXICO CITY -- About 800 fishermen in the northernmost crook of the Gulf of California have taken up the government's offer of payments to stop fishing with nets and, in some cases, to stop fishing altogether, Mexican conservationists said on Tuesday.

The offer is intended to save a small porpoise that is threatened with extinction as an unintended byproduct of commercial fishing. The porpoise, called a vaquita, is often trapped and killed in the gill nets that fishermen use to catch shrimp, mackerel and sharks.

Probably no more than 150 vaquitas survive, conservationists say. The population could fall to 100 in a couple of years. If that occurred, there would be too few sexually mature adults left for the species to recover.

"We have one or two years," said Omar Vidal, the director of the World Wildlife Fund in Mexico and a biologist who has studied the vaquita for 25 years. "We're on the brink."

The Mexican government agrees. It has spent about $20 million over the last two years on conservation measures, primarily to persuade 800 of the 4,000 registered fishermen in the area to accept its offer to stop using nets or to cease fishing entirely, according to the environment minister, Juan Elvira Quesada. Next year, officials hope to spend an additional $13 million to continue the plan.

Many of the fishermen who have accepted the offer will use the money to start businesses. For those fishermen reluctant to give up their livelihood, there is a new net, developed with the help of the World Wildlife Fund, that does not trap the vaquita.

With dark doe eyes and pale skin, the vaquita looks as if it had been drawn by a child who outlined the eyes and mouth in black felt-tip pen. It inhabits the Gulf of California's shallower waters.

The conservationists' sense of urgency is driven by a sad precedent. Last year, a cousin of the vaquita, the Chinese river dolphin, was declared extinct.